OpenERP sits in between your users and the database. There is no particular reason why 8069 is used. If you want to use port 80, then your OpenERP installation will look to your users like an ordinary website. On its back end OpenERP must connect to a PostgreSQL database, in order to make your user's data modifications permanent. The standard port for that is 5432.

Often the database is installed on an entirely separate physical machine, that has no Internet visibility at all, and may be configured to accept connections from no where else but the specific outgoing port and IP address, of the OpenERP server on the front end.